The environmental authorities are placing ever more stringent demands on the pulp industry to decrease the use of chemicals which can be harmful to the environment, such as chlorine, for example. Thus, permitted discharges of organic chlorine compounds in the effluent water from bleaching plants and the subsequent cooking process have been successively decreased and are now at such a low level that pulp works have in many cases stopped using organic chlorine compounds as bleaching agents. In addition, market forces are tending successively to increase the demand for paper products which have not been bleached with chlorine.
The pulp industry is therefore seeking methods which permit bleaching of pulp without using these chemicals. As an example of such a method, the lignox method (see SE-A 8902058) can be mentioned, in which, inter alia, bleaching is carried out with hydrogen peroxide. Ozone is another bleaching chemical of interest which is also being used to an increasing extent. It is thus possible, using such bleaching chemicals, to achieve the brightnesses which are demanded for marketed pulp, i.e. 89 ISO and higher, without using chlorine-containing bleaching agents.
However, there is a problem in employing these bleaching chemicals which do not contain chlorine in currently known bleaching processes, namely that these chemicals impair to a relatively large extent the quality of the pulp fibers.